Cannot access any other .htm page on .www

I am new to Web Design and been suggested this site. I tried to upload all my files. I have a main.htm web page and have other sub .htm pages linked to the main page along with images.

I could successfully load the main.htm but when I clicked on the sub pages they wouldn't open.

I even tried to put everything i.e. sub.htm and images onto a folder and pasted it on the .www page but when I called the main page on the Host Server it still wouldn't open the linked images nor the sub.htm pages from the main page.